Current Traffic Alert Collision Avoidance (TCAS) installations are expensive. Dual TCAS installations are even more so because they require an extensive amount of cable and/or coaxial cable switches. Standard TCAS antennas also do not typically have a wide enough bandwidth to provide for integration of all L-band avionics equipment such as TCAS, Distance Measuring Equipment (DME) and transponder (XPDR) using the same antennas. U.S. Pat. No. 4,855,748, incorporated herein by reference describes a system and method for determining TCAS bearing estimation using a four element planar array antenna. TCAS installations typically use two such four element antennas, one on the top surface of an aircraft, and the other on the bottom surface of the aircraft. This typically requires a coaxial cable connection for each antenna element, resulting in at least eight total cables and an extensive number of coaxial cable switches. This results in considerable complexity and expense. U.S. Pat. No. 6,222,480, incorporated herein by reference, describes a system and method that integrates TCAS and XPDR functions. However, the system and method described in the '480 patent use a top four element antenna and a bottom four element antenna that results in extensive cable and coaxial switch requirements. Accordingly, there is a need for a TCAS antenna system that uses less cable and coaxial switches and is thus less expensive.
The '748 and '480 patents also use a single transmitting, receiving, and processing unit. This has the disadvantage of not offering any redundancy should the transmitting, receiving, and processing unit become damaged or malfunction. Accordingly, there is a further need for a TCAS system offering some level of redundancy.